The present invention relates generally to antennas and specifically to antennas designed for maximum magnetic radiation with minimum electrostatic radiation.
When current flows through a wire, two separate and distinct fields are created. An electrostatic field, and a magnetic field together form an electromagnetic field which is the basis for radio and television transmissions. The Federal Communications Commission (FCC) regulates the radiation of electromagnetic fields (by setting maximum permissible levels of radiation 100 feet from the unlicensed transmitter) in order to provide some order in the utilization of the various radio frequencies. Because the electrostatic or magnetic fields separately do not lend themselves to propagation over extended distances, there is no Federal regulation of these fields.
It has recently become advantageous to utilize electromagnetic fields to provide security in libraries, stores, etc. Generally, an article to be protected has a coil, foil strip, or some other security device which, in response to a changing electromagnetic field, resonates and provides a change in the field which is then detected. Thus, when the article with the security device mounted thereon is brought into close proximity to the electromagnetic transmitter/receiver antenna, the change in the field can be detected and consequently sets off an alarm. A further discussion of the history of a typical security system is set forth in U.S. Pat. No. 4,074,249 to Minasy and in U.S. Pat. No. Re. 29,610, reissued to Minasy et al., said patents herein incorporated by reference.
It is desirable where such a security detection system is utilized in a department store, to have the antennas as inobtrusive as possible so that customers feel free to walk into and out of the store without fear of being falsely detected. On the other hand, it is desirable to have complete security coverage in terms of the antenna radiation pattern of the entry way in order to detect whether items are being surreptitiously removed from the store.
One solution to the problem of ensuring adequate electromagnetic radiation for detection of the security device with a limited antenna size is to increase the strength of the electromagnetic radiation field. However, this results in increased radiation which may rise above the FCC permissible maximum for non-licensed transmitters. Accordingly, to exceed these maximum limits, exemptions from the FCC regulations are required. Most recently, several of the major manufacturers of such security systems applied successfully for exemptions from the electromagnetic field generation requirements based on the premise that they could not otherwise properly operate their security system.
A further problem is that it is desirable to have the receiving antenna extremely sensitive to extremely weak electromagnetic signals in the immediate vicinity of the antenna but essentially non-responsive to electromagnetic signals generated some distance away from the antenna (far-field radiation). Prior art security systems have dealt with this problem by having extremely narrow bandpass receivers such that the receiver is responsive only to a single small band of frequencies thus cutting down the likelihood of spurious electromagnetic radiation setting off the alarm system inadvertently.
Another difficulty with prior art antenna systems such as that shown in U.S. Pat. No. 4,016,553 to Novikoff et al., is that although the antennas are connected in bucking relationship (to reduce far-field radiation), the size necessary to so orient the two or three antenna elements requires structures approximately a foot wide and a foot deep on each side of the opening which is to be protected. A further problem is that even this two or three element antenna does not provide strong electromagnetic fields in all planes and thus the performance of this security system is seriously degraded in certain planes. In this particular system, the orientation of a resonant tag security device in a horizontal plane would essentially be undetected.
Finally, many of the above-described antennae are sensitive to the human touch to the point that the alarm will be activated when one or both antennae are touched by hand. Additionally, when an electrostatic shield is made up of separate metal sections there is a problem with contact.